Star Clusters in M31: I. A Catalog and a Study of the Young Clusters

We present an updated catalog of 1300 objects in the field of M31, including 670 likely star clusters of various types. Archival images were inspected to confirm cluster classifications where possible, but most of the classifications were based on spectra taken of ~1000 objects with the Hectospec fiber positioner and spectrograph on the 6.5m MMT. The spectra and images of young clusters are analyzed in detail in this paper. Ages, reddenings and masses of 140 young clusters are derived by comparing the observed spectra and photometry with model spectra. We find these to have masses as great as 10^5 with a median of 10^4 M_sun, and a median age of 0.25 Gyr. Thus these clusters are similar in mass to the LMC young massive clusters, and are in between Milky Way open clusters and the globulars of M31 and the Milky Way. Most of the M31 young clusters have the low concentration typical of Milky Way open clusters, and we expect that most of these will be disrupted in the next Gyr, but a few have high concentrations and will likely survive longer. The spatial distribution of the young clusters is well correlated with the star-forming regions as mapped out by mid-IR emission. A kinematic analysis likewise confirms the spatial association of the young clusters with the young disk in M31.Comment: 18 pages of text and figures, 23 of tables, to appear in AJ. http://www.cfa.harvard.edu/oir/eg/m31clusters/M31_Hectospec.html is a new M31 web site containing tables and postage stamp images of the entire catalo

Internal dynamics of the radio-halo cluster A2744

We present a detailed dynamical analysis of the rich galaxy cluster A2744, containing a powerful diffuse radio halo.Our analysis is based on redshift data for 102 galaxies, part of them recovered from unexplored spectra in the ESO archive. We combine galaxy velocity and position information to select the cluster members and determine global dynamical properties of the cluster. We use a variety of statistical tests to detect possible substructures. We find that A2744 appears as a well isolated peak in the redshift space at =0.306, which includes 85 galaxies recognized as cluster members. We compute the line-of-sight (LOS) velocity dispersion of galaxies (~1750 km/sec), which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 8 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non-Gaussian nature of the velocity distribution, the presence of a velocity gradient and a significant substructure. In particular, our results suggest a merging scenario of two clumps with a mass ratio of 3:1 and a LOS impact velocity (rest frame) of ~3000 km/sec, likely observed just after the core passage. The merging is occuring roughly in the NS direction with the axis close to the LOS. This scenario agrees with that proposed on the basis of recent Chandra results in its general lines, although suggesting a somewhat more advanced merging phase.Comment: 14 pages. Paper in press on Astronomy & Astrophysic

Substructures in WINGS clusters

We search for and characterize substructures in the projected distribution of galaxies observed in the wide field CCD images of the 77 nearby clusters of the WIde-field Nearby Galaxy-cluster Survey (WINGS). This sample is complete in X-ray flux in the redshift range 0.04<z<0.07. We search for substructures in WINGS clusters with DEDICA, an adaptive-kernel procedure. We test the procedure on Monte-Carlo simulations of the observed frames and determine the reliability for the detected structures. DEDICA identifies at least one reliable structure in the field of 55 clusters. 40 of these clusters have a total of 69 substructures at the same redshift of the cluster (redshift estimates of substructures are from color-magnitude diagrams). The fraction of clusters with subclusters (73%) is higher than in most studies. The presence of subclusters affects the relative luminosities of the brightest cluster galaxies (BCGs). Down to L ~ 10^11.2 L_Sun, our observed differential distribution of subcluster luminosities is consistent with the theoretical prediction of the differential mass function of substructures in cosmological simulations.Comment: A&A accepted - figure 6 is available from http://adlibitum.oats.inaf.it/ramella/WINGSfig

Internal dynamics of the radio halo cluster Abell 773: a multiwavelength analysis

We conduct an intensive study of the rich, X-ray luminous galaxy cluster A773 at z=0.22 containing a diffuse radio halo to determine its dynamical status. Our analysis is based on new spectroscopic data obtained at the TNG telescope for 107 galaxies, 37 spectra recovered from the CFHT archive and new photometric data obtained at the INT telescope. We use statistical tools to select 100 cluster members (out to ~1.8 Mpc from the cluster centre), to analyse the kinematics of cluster galaxies and to determine the cluster structure. Our analysis is also performed by using X-ray data stored in the Chandra archive. Results: The 2D distribution shows two significant peaks separated by ~2 arcmin in the EW direction with the main western one closely located at the position of the two dominant galaxies and the X-ray peak. The velocity distribution of cluster galaxies shows two peaks at v~65000 and ~67500 km/s, corresponding to the velocities of the two dominant galaxies. The low velocity structure has a high velocity dispersion sigma_v=800-1100 km/s and its galaxies are centred on the western 2D peak. The high velocity structure has intermediate velocity dispersion sigma_v~500 km/s and is characterized by a complex 2D structure with a component centred on the western 2D peak and a component centred on the eastern 2D peak, these components probably being two independent groups.We estimate a cluster mass within 1 Mpc of 6--11 e^14 h_70 Msun. Our analysis of Chandra data shows the presence of two very close peaks in the core and the elongation of the X-ray emission in the NEE--SWW direction. Our results suggest we are looking at probably two groups in an advanced stage of merging. In particular, the radio halo seems to be related to the merger of the eastern group.Comment: Accepted for publication in in A&

The Double Galaxy Cluster Abell 2465 I. Basic Properties: Optical Imaging and Spectroscopy

Optical imaging and spectroscopic observations of the z = 0.245 double galaxy cluster Abell 2465 are described. This object appears to be undergoing a major merger. It is a double X-ray source and is detected in the radio at 1.4 GHz. This paper investigates signatures of the interaction of the two components. Redshifts were measured to determine velocity dispersions and virial radii of each component. The technique of fuzzy clustering was used to assign membership weights to the galaxies in each clump. Using redshifts of 93 cluster members within 1.4 Mpc of the subcluster centres, the virial masses and anisotropy parameters are derived. 37% of the spectroscopically observed galaxies show emission lines and are predominantly star forming in the diagnostic diagram. No strong AGN sources were found. The emission line galaxies tend to lie between the two cluster centres with more near the SW clump. The luminosity functions of the two subclusters differ. The NE component is similar to many rich clusters, while the SW component has more faint galaxies. The NE clump's light profile follows a single NFW profile with c = 10 while the SW is better fit with an extended outer region and a compact inner core, consistent with available X-ray data indicating that the SW clump has a cooling core. The observed differences and properties of the two components of Abell 2465 are interpreted to have been caused by a collision 2-4 Gyr ago, after which they have moved apart and are now near their apocentres, although the start of a merger remains a possibility. The number of emission line galaxies gives weight to the idea that galaxy cluster collisions trigger star formation.Comment: 21 pages, 18 Figures Replaced typos, mostly in references To appear in MNRAS, Accepted 2010 December 16. Received 2010 December 15; in original form 2010 November 0